I see a kid from nowhere arriving between the girl from the shop who called him by -chan and calling the guy "Daddy".
If she wasn't their daughter, why the fuck did she appear at all?
As for the manga: Okay, there's a lot of fucking unrealistic points in it. Why would a 20 years old marry a 50 years old, why would she stay to cure his hikki son who never talked to her, why has she left the chance of her life run away to stay with the hikki.
But for the moral of the history, I think it's still quite realist. If you're gonna stay in your room, you're never gonna know what it is to live and have human contact. NEVER. But if you do the effort to go out and see the world, not everything will be nice, in fact a lot of things are gonna suck, but there's still good things in it.